The modifications I've made to iGear's MW-03 "Hench" have prompted a lot of questions, and a little confusion...

I've prepared this thread to illustrate the process, but I didn't do a great job of documenting the work I did...
I sort of made it up as I went along, and didn't stop to take many pictures along the way.
I've therefore done my best to reconstruct the process with photos taken after the fact.

It's not a particularly complicated mod' -- in fact, it probably took more time and effort to prepare this thread! --
but I've tried to explain it in as much detail as possible.
What you'll need:

A small Phillips-head screwdriver
A modeling saw, or Dremel with a cutting wheel
A hobby knife or X-Acto for fine trimming
Model glue or Krazy glue
A plastic tab 10mm square and 2mm thick
A 5mm post at least 25mm long
What might also come in handy:

Before I even got Hench in hand, I knew I'd have to make some modifications.
The figure's a terrific depiction of G1 Brawn, with a lot of design cues taken from Floro Dery's animation model;
However, all the kibble around the head needed to be repositioned.

Our goal is to separate the yellow body of the robot from the green body of the vehicle.
First, three screws need taking out: two behind covers above the head, and one through the spare tire on the roof.

With the yellow part separated from the green, start by cutting off the frame around the head:

I did a messy job, but I wanted to be careful not to damage the shoulder joints.
Since the top of the torso is highly visible in robot mode, I sanded the shoulders smooth to leave no trace of the cut:

Note that I also cut off the rod that ran through the center of the torso; that could get in the way, so it should be removed.

There are also protrusions on the green section that will get in the way, but cut them off carefully;
They’ll be needed to hold the figure together later.

This top portion, for instance, holds the head and body together:

Cut it flush with the plane of the green back, where the green arrows indicate.

When gluing it to the yellow torso, make sure glue only gets on the outside edges (indicated in green);
If any glue gets in the socket where the neck rests, you won't be able to turn the head.

The back of the head was popped out to repaint the eyes; they're on stalks, like any Hasbro light-pipe.
This also made it easier to repaint the face, which should be a lighter silver than the helmet around it.

The lower back protrusion is equally important to cut off cleanly, as it will hold the torso and pelvis together.
With both green protrusions separated from the back, it should now look like this:

Note the cuts (indicated with green arrows) have been trimmed and sanded to be flush with the side walls.

Now that we've severed the permanent connections between vehicle and robot halves, we need new attachment points.
To hold the front and back halves together in vehicle mode, we need a tab of these approximate dimensions:

The tab should be sandwiched halfway into the open space in the lower back protrusion we cut off earlier.
It will fit into the matching cavity on the back piece to connect the yellow and green halves of the figure together.

The lower back protrusion is what connects the pelvis to the torso, so it needs to be glued in place;
Again, be careful to apply the glue only to the surfaces indicated in green:

If you get any glue between the green part and the pelvis (where the red arrow's pointing), pull those pieces apart at once;
You’ll have to wipe off the glue, sand the parts down and try again, or else the waist won't turn and Brawn won't transform.

Here's a clearer angle showing where the tab was installed, and where it sits in relation to the waist joint:

...and the same image, with arrows indicating where to glue and where not to.

If you've succeeded in getting a snug fit, the front half can be aligned with the back half just as it was originally:

Transformation to vehicle mode is achieved exactly as before.

Now, we need to engineer a second attachment point, so that the back half will sit lower in robot mode:

This will require a 5mm rod that runs through the spare tire and plugs into the back of the pelvis piece.

Any rod of the right length and width will do (in fact, I used a chopstick to test the alignment initially), but here's a quick solution:

The section in the middle is what I used. The soft green tip provides a secure fit that pops in and out easily for transformation.
I drilled through the shaft of the spare tire with a 5mm bit to provide a secure connection to the post, and glued it firmly in place:

Glue the spare tire back onto the top of the vehicle shell, and the post will protrude from the bottom.

Almost finished!
All that's left is to cut shallow grooves into the yellow portion on the back, to align the back kibble with the shoulder joints.
The green arrows indicate where I cut; two identical triangles at right-angles:

If you've cut in the right places, it all fits together very neatly.

Finally, to give an additional modern touch to the figure, I replaced iGear's shotgun with Classics Sideswipe's pistol:

(Classics Sunstreaker, Red Alert, and TFCC's Punch/Counterpunch also come with the same weapon).
It can be concealed under the chassis in vehicle mode like a secret weapon:

It plugs into the same glorious hole we use to hold the figure together in robot mode!

Takara's Henkei! vac-metalized version looks even better, with Brawn's exhaust pipes a natural extension of the muffler...